This code was copy-pasted at some point. Revisit it to reunify it.
~~Doing that then encouraged simplifying the types of a couple of
related functions.~~
~~As a follow-up, move two helper functions, `isReadmeFile()` and
`isReadmeFileExtension()`, intimately tied to `findReadmeFile()`, in as
package-private.~~
Signed-off-by: Nick Guenther <nick.guenther@polymtl.ca>